You can certainly make sure your mail settings in Outlook are not outdated. By that I mean use the recommended TalkTalk email settings Email settings - IMAP or POP3 and in particular use SSL/TLS encryption.
For mail on the TalkTalk Mail platform, so that's talktalk.net, lineone.net, tiscali.co.uk and others in the tiscali family the outgoing server uses STARTTLS connection security, port 587 and normal password for authentication along with using the Outlook setting 'my outgoing server requires authentication'.
I'd try to send via webmail so sign in to TalkTalk Mail here and send to just one BT Mail recipient to check.
I thought this problem was sorted but now it has recurred. Only with btinternet addresses! I managed to get a chat with Talktalk who promised to do some diagnostics and reply to me in 72 hours but not heard a thing and problem still occurring. Does anyone else have this problem with btinternet. The return message always states queue time exceeded